1. Field of the Invention
The present invention relates to an optical pickup apparatus and a beam splitter, for use in information recording on or reproducing information from or reproducing information from an optical disk.
2. Description of the Related Art
An optical pickup apparatus is used for recording information on or reproducing information from an optical disk. The optical disk includes a compact disc (abbreviated as CD), a digital versatile disc (abbreviated as DVD) and a Blu-ray disc (registered trademark). A recording/reproducing process of the CD is executed using laser light in an infrared wavelength region around 780 nm. A recording/reproducing process of the DVD is executed using laser light of which wavelength is shorter than that of the laser light used for the recording/reproducing process of the CD; to be specific, red light in a wavelength region around 650 nm. A recording/reproducing process of the Blu-ray disc is executed using laser light of which wavelength is even shorter than that of the laser light used for recording/reproducing process of the DVD; to be specific, blue-violet light in a wavelength region around 405 nm.
In the optical disk, a recording layer is formed. In recording information or reproducing information from the optical disk as mentioned above, the recording layer of the optical disk is irradiated with condensed laser light, and light reflected by the optical disk is then received by a light-receiving unit. By so doing, the information is recorded or reproduced and moreover, a servo control is carried out by detecting a servo signal including a focusing error signal and a tracking error signal.
FIG. 14 is a schematic view showing a configuration of an optical pickup apparatus 1 of related art. In the optical pickup apparatus 1 of related art, light emitted from a light source 2 is split by a diffraction grating 3 into a main beam and a pair of sub beams, which are then transmitted through a beam splitter 4 and converted by a collimation lens 5 into parallel light, thereafter being condensed by an objective lens 6 onto a recording layer 8 of an optical disk 7. Light reflected by the recording layer 8 of the optical disk 7 is then transmitted by the objective lens 6 and the collimation lens 5, thereafter being split and condensed by the beam splitter 4 to be thus received by a light-receiving unit 9. The optical pickup apparatus 1 of related art as just described are disclosed in Japanese Unexamined Patent Publications JP-A 2004-288227 and JP-A 2004-303296.
FIG. 15 is a view showing one example of the beam splitter 4. In the example, the beam splitter 4 is given an additional reference symbol “a”, and the beam splitter 4a is thus shown. FIG. 15 shows a relation between the beam splitter 4a and the light-receiving unit 9. The light-receiving unit 9 has a light-receiving section 11 for focusing which is used for producing a focusing error signal, and a pair of light-receiving sections 12a and 12b for tracking, each of which is used for producing a tracking error signal.
The beam splitter 4a is achieved by a hologram element. The beam splitter 4a includes a splitting region 13 for focusing where the light reflected by the recording layer 8 of the optical disc 7 is diffracted so as to fall on the light-receiving section 11 for focusing, and a pair of splitting regions 14a and 14b for tracking where the light reflected by the recording layer 8 of the optical disc 7 is diffracted so as to fall on the respective light-receiving sections 12a and 12b for tracking.
The splitting region 13 for focusing is bounded by the pair of splitting regions 14a and 14b for tracking at a boundary line 15 which is schematically drawn perpendicular to a reference optical axis L of the optical system and along a virtual line 16 extending in a tracking direction T1. Of the boundary line 15, a part 17 is located radially outward of the reference optical axis L of the optical system. The part 17 is formed along the virtual line 16. Of the boundary line 15, a part 18 is located radially inward of the reference optical axis L of the optical system. The part 18 is out of the virtual line 16 so that the splitting region 13 for focusing is larger than a splitting region 13 for focusing which is bounded with use of a part 18 extending along the virtual line 16.
FIG. 16 is a view showing another example of the beam splitter 4. In the example, the beam splitter 4 is given an additional reference symbol “b”, and the beam splitter 4b is thus shown. FIG. 16 shows a relation between the beam splitter 4b and the light-receiving unit 9. The beam splitter 4b of the present example is similar to the above-described beam splitter 4a shown as one example in FIG. 15, and only the points different from the beam splitter 4a will be therefore described. In the beam splitter 4b of the example, the part 17 of boundary line 15 located radially outward of the reference optical axis L of the optical system is out of the virtual line 16 so that the pair of the splitting regions 14a and 14b for tracking is larger than a pair of splitting regions 14a and 14b for tracking which is bounded with use of a part 17 extending along the virtual line 16.
In some of the optical disk 7, a plurality of the recording layers 8 are stacked one after the other in a thickness-wise direction of the optical disk 7 in order to increase a recording capacity. In recording information on or reproducing information from the optical disk 7 as described above, the light-receiving unit 9 receives not only the light reflected by a light-condensing recording layer 8a where light emitted from the light source 2 is condensed, but also the light reflected by a non-light-condensing recording layer 8b which is different from the light-condensing recording layer 8b. 
FIG. 17 is a view of assistance in briefly explaining how the optical disk 7 having two recording layers 8 transmits and reflects light. In FIG. 17, it is assumed that the non-light-condensing recording layer 8b is located behind the light-condensing recording layer 8a when the optical disk 7 is viewed from the light source 2. Light 21 emitted from the light source 2 is condensed and reflected on the light-condensing recording layer 8a. A part of the light emitted from the light source 2 is transmitted by the light-condensing recording layer 8a. Light 22 transmitted by the light-condensing recording layer 8a is reflected on the non-light-condensing recording layer 8b as if the light 22 is focused on a virtual focus point 23 behind the non-light-condensing recording layer 8b when viewed from the light source 23.
FIG. 18 is a view illustrating how the light reflected by the non-light-condensing recording layer 8b is received by the light-receiving unit 9 through the beam splitter 4a shown in FIG. 15. FIG. 19 is a view illustrating how the light reflected by the non-light-condensing recording layer 8b is received by the light-receiving portion 9 through the beam splitter 4b shown in FIG. 16. In FIG. 18 and FIG. 19, it is assumed that the non-light-condensing recording layer 8b is located behind the light-condensing recording layer 8a when the optical disk 7 is viewed from the light source 2. In this case, light 25 reflected by the non-light-condensing recording layer 8b enters the light-receiving section 11 for focusing and respective light-receiving sections 12a and 12b for tracking more spreadingly than light reflected by the light-condensing recording layer 8a, and is received thereby. This poses a problem that the servo signal is adversely influenced.